The present invention relates generally to “audio ground switches” that include depletion mode MOS transistors which function as audio ground switches connected to prevent build-up of charge that may result in discharging and associated popping sounds when a headset is plugged into an audio signal jack of a device generating the audio signal.
The discharge of the above-mentioned charge build-up occurs because an audio ground switch, along with circuitry connected to it, have parasitic capacitances, inductances, and resistances that are subject to build up of static charge which can be discharged similarly to common electrostatic discharge (ESD). The above-mentioned built-up charge in audio ground switch circuitry may be electrostatically discharged to ground at the instant a headset is connected to circuitry connected to the audio ground switch. The resulting current through the headset speaker resistance may cause the above-mentioned clicking/popping sounds.
Referring to FIG. 1, a conventional audio ground switch circuit 1 is implemented in an integrated circuit chip that includes a conventional charge pump 2 coupled between a positive supply voltage VDD and a system ground. Charge pump 2 produces an output voltage VCP on conductor 3. Charge pump 2 includes conventional internal circuitry that discharges VCP to zero volts if VDD falls below an under-voltage “lockout threshold” voltage. The charge pump output voltage VCP is connected by conductor 3 to the gate electrodes of P-channel MOS (metal oxide semiconductor) depletion mode field effect transistors MP1 and MP2 which normally are in their conductive ON states when VCP is at zero volts. When VDD is at a normal level, for example 3.3 volts, then charge pump output voltage VCP is at a boosted level, for example 7 volts. The source of depletion mode ground switch transistor MP1 is connected to a conductor 6-1 having an audio information signal voltage VOUT1 thereon, and the source of depletion mode ground switch transistor MP2 is connected to a conductor 6-2 having an audio information signal voltage VOUT2 thereon. The source electrodes of depletion mode transistors MP1 and MP2 are limited to a sufficiently low voltage that, combined with the boosted charge pump voltage VCP, they produce a sufficiently large gate-to-source reverse bias voltage VGS to change the state of depletion mode transistors MP1 and MP2 from their conductive regions of operation to their cutoff regions.
FIG. 2 shows the connections and signals associated with depletion mode transistor MP1 in more detail for the case in which VDD is equal to zero. Depletion mode transistor MP1 has P-type source and drain regions formed in an N-type well region 4 of depletion mode transistor MP1. The P-type source and the N-type well region form an associated parasitic diode D1 which has a substantial parasitic capacitance, and the P-type drain and the N-type well region 4 of depletion mode transistor MP1 form an associated parasitic diode D2 which also has a substantial parasitic capacitance. The N-type well region 4 is formed on a P-type substrate and together they form an associated parasitic substrate diode D3. (See the integrated circuit section view of depletion mode transistor MP1 in subsequently described FIG. 5.) If MP1 is “open” in its high impedance OFF state, a large negative voltage on conductor 6-1 will certainly forward bias substrate diode D3-1, but that is not the case if MP1 is conductive and therefore acting like a ground switch. If MP1 is in its conductive ON state, it is unlikely that any audio signal would be present because it would not be able to be developed across the parallel combination of the ground resistor R1-1 and the low channel resistance of MP1, which might be somewhere between 0.1 and 1.0 ohms.
A relatively large-amplitude audio signal having a range of, for example, ±2.63 volts may be produced on conductor 7 by an audio amplifier 8 in a conventional CODEC (coder-decoder) 11. That audio signal is coupled across a resistive voltage divider including a 16 ohm headset resistance R2 (of a headset 13) coupled between the output 7 of audio amplifier 8 and VOUT1 conductor 6-1 and a 7 ohm “ground resistor” R1 coupled between VOUT1 conductor 6-1 and ground. Audio amplifier 8 also is referenced to ground. (Audio engineers sometimes connect a “ground resistor” such as resistor R1 in series with the system ground to reduce or eliminate so-called “ground noise”.) The divided-down output signal produced by audio amplifier 8 appears as VOUT1 on conductor 6-1 and would have a range of ±0.8 volts if depletion mode transistor MP1 in FIG. 2 were OFF instead of ON. However, since depletion mode transistor MP1 in FIG. 2 is in its ON condition, its very low channel resistance is in parallel with ground resistor R1 and causes VOUT1 to be essentially equal to zero. Note however, that audio signals usually are not present while the depletion mode field effect transistor MP1 is in its conductive or ON state. The signal on conductor 6-1 is typically used to provide internal compensation in audio CODEC 11.
FIG. 3 shows the same structure shown in FIG. 2, but in this case VDD is not equal to zero. Instead, VDD has a sufficiently large value to cause charge pump output voltage VCP to be equal to approximately +7 volts, which results in a magnitude of the gate-to-source voltage (VGS) of depletion mode transistor MP1 sufficiently high to switch depletion mode transistor MP1 completely OFF into its high-impedance state. In this case the audio signal on amplifier output conductor 7 typically is present, so the full ±0.8 volt output value of VOUT1 is produced on conductor 6-1 by the voltage division of the audio amplifier output voltage on conductor 7 by the headphone resistance R2 and the ground resistance R1. The same circuitry shown for depletion mode transistor MP1 in FIGS. 2 and 3 can, of course, also be utilized for depletion mode transistor MP2 in FIG. 1.
The prior art ground switch integrated circuit of Prior Art FIGS. 1-3 provide a very low resistance in parallel with ground resistor R1 when no power is being applied to the charge pump 2 (i.e., when VDD=0), and ground switch integrated circuit of FIGS. 1-3 also turns the depletion mode transistor MP1 off whenever adequate VDD power is being applied to charge pump 2 such that audio information VOUT1 can be produced on conductor 6-1 and applied to the ground sensing input of audio CODEC 11 for processing.
The direct connection of N-type well 4 to VOUT1 conductor 6-1 prevents forward biasing of parasitic diode D1 including the PN junction between the P-type source and the N-type well region 4 of depletion mode transistor MP1 when VOUT1 is positive. Unfortunately, if depletion mode transistor MP1 is switched into its high impedance OFF state, then the −0.8 volt portion of the AC signal VOUT1 on conductor 6-1 may cause parasitic diodes D2 and D3 to become forward biased, and that introduces a large amount of distortion into the system audio signal VOUT1.
If speaker or headset 13 is plugged into the headset jack of a personal computer or the like when depletion mode transistor MP1 is OFF, and if the audio volume is turned up to its maximum level during a “no audio event” (i.e., when no desired audio signal such as a music signal is being provided), an annoying audio frequency ground noise signal or “audio frequency hum” may be heard from the headset speaker resistance represented by resistor R2. The 7 ohm noise reduction resistor R1 in the ground path and the headset resistance R2 function together as a voltage divider that reduces the magnitude of the maximum negative voltage swing of VOUT1 (−0.8 volts in this example) in order to prevent forward biasing of the substrate diode D3 formed by the P-type source and the diode D2 formed by N-type well region 4 of depletion mode transistor MP1. The 7 ohm ground resistor R1 reduces the audio hum amplitude during such a “no audio” event. The R1 ground sense resistor is needed to provide a ground sense input signal to the CODEC 11. This ground sense input signal is then used to cancel the noise of the ground signal. The need for this function causes the relatively large-magnitude signal of +/−0.8 volts to appear on VOUT1 conductor 6-1.
Typically, there will be some charge buildup on the parasitic capacitances associated with the ground conductor and/or the audio signal conductor 6-1 when no VDD power is being applied to charge pump 2. The main reason for requiring audio ground switches is to provide resistive paths for discharge of such charge buildup is resulting from plugging a headset into a headset jack to receive the audio signal VOUT1 to thereby prevent a sudden electrical discharge through the headset resistance R2 and thereby preventing the annoying clicking/popping sounds.
When the audio signal voltage VOUT1 is present on conductor 6-1 and is applied to the drain of depletion mode ground switch transistor MP1 in FIGS. 1-3, then a −0.8 volt value of VOUT1 on conductor 6-1 appears on the cathode of substrate diode D3, forward biasing it and causing a large amount of distortion in VOUT1. The main functions of the depletion mode transistor MP1 in its ON state are to prevent static charge buildup and to dampen or slow down any discharge of built-up static charge when the headset 13 is plugged in.
Thus, there is an unmet need for improved audio ground switch circuitry that prevents buildup of charge and thereby prevent subsequent discharge thereof in depletion mode ground switch transistors under all operating conditions without compromising the quality of desired audio signals that are present.
There also is an unmet need for improved audio ground switch circuitry that prevents negative portions of an audio signal from causing ground switch circuitry receiving the audio signal to produce distortion in the audio signal.
There also is an unmet need for improved audio ground switch circuitry that prevents popping sounds caused by discharging of charge built up in the audio ground switch circuitry at the instant at which a headset is plugged into a jack receiving an audio signal.
There also is an unmet need for improved audio ground switch circuitry that enables use of higher resistance ground resistors without causing distortion of an audio signal received by the audio ground switch circuitry.